This invention relates to a porous, microscopically transparent membrane having bulk properties which differ from its surface properties and to a process for making the same. More particularly, this invention relates to a transparent microporous or ultrafiltration membrane formed from a polytetrafluoroethylene substrate wherein the substrate is hydrophobic and opaque or translucent in its unmodified form and is rendered hydrophilic and transparent when wet with aqueous media and to a process for forming such a membrane. This invention also relates to a device utilizing the membrane of this invention for observing cell growth.
As used herein, the term "transparent" means microscopically transparent. That is, the membrane is transparent to the extent that normal sized cells, e.g., 5 to 30 microns, can be viewed through the membrane wall with a microscope such as at 50 times to 600 times magnification. The transparency can be specifically quantified by measuring optical density with visible light such as with a spectrophotometer. The optical density of the wet membrane of this invention is between 0 and 0.5, preferably 0 to 0.3 when measured with a spectrophotometer at 410 nanometers visible light.
In many applications of membrane technology, it is desirable to utilize a membrane filter which is mechanically strong, is thermally stable, is relatively inert chemically and is insoluble in most organic solvents. Often, it is desirable that the membrane have surface properties which are radically different from and sometimes incompatible with the bulk properties set forth above. In some instances it is desired to form a porous membrane which is transparent and which is capable of supporting cell growth in order that cell growth can be easily monitored merely by microscopic examination of the live cells through the membrane. For example, in cell growth technology it is desirable to effect cell growth on and within the pores of a membrane rather than on a flat surface so that three dimensional cell growth rather than two dimensional cell growth can be effected. This is particularly true when growing epithelial cells such as those derived from the lungs, kidneys, or intestine which demonstrate a distinct polarity. Prior to the present invention, porous membranes used for cell growth are opaque or translucent so that the progress of cell growth cannot be viewed. At the present time, it is necessary to take a subsample of the cells under conditions toxic to the cells in order to ascertain the level of cell growth. This is undesirable since it reduces the number of cells available for producing the desired cell product or cell marker. Therefore, it would be desirable to provide a transparent membrane capable of permitting cell growth under conditions that the extent of cell growth can be observed visually.
It is known that polytetrafluoroethylene can be rendered transparent in water by immersing it in 100% methanol followed by immersing it sequentially into methanol-water mixtures and lastly in 100% water. However, the polytetrafluoroethylene surface is not rendered hydrophilic by such a wetting procedure. In addition, this procedure is inconvenient for the end user and the membrane cannot be rewet if allowed to dry.
Conventional methodology presently used to achieve the duality of function of bulk properties which differ from the surface properties is to coat a preformed membrane having the desired bulk properties with an oligomer or polymer having the desired surface properties. Typical coating materials include surfactants, many of which are toxic to cells, and water soluble polymers such as polyvinylpyrrolidone. This approach to modifying surface properties is undesirable since the coating is only temporary and exposure to any process fluid, particularly when the substrate having the desired bulk properties is a porous membrane, effects removal of the coating from the porous membrane. Membranes treated in this fashion cannot be steam sterilized, cannot be rewet once dried after being wetted with water and exhibit high extractable levels. These properties are unacceptable in many filtration applications, particularly when processing biological fluids which are to be sterilized or subsequently analyzed. This is particularly true in cell culture since many of these extractables are cytotoxic and, therefore, incompatible with cell growth.
It also has been proposed to utilize graft polymerization techniques to modify the surface characteristics of a polymeric substrate. Typical examples of graft polymerization are shown for example in U.S. Pat. Nos. 3,253,057; 4,151,225; 4,278,777 and 4,311,573. It is difficult to utilize presently available graft polymerization techniques to modify the surface properties of the porous membrane. This is because it is difficult to modify the entire surface of the membrane including the surfaces within the pores while avoiding pore blockage and while retaining membrane porosity In U.S. Pat. No. 4,340,482, issued July 20, 1982, it has been proposed to modify the surface of porous membranes formed from hydrophobic fluorine-containing polymers by binding a primary amine such as glycine to the hydrophobic substrate. The primary amine renders the polymer surface hydrophilic and can be utilized as a reactant site to link a polymerizable monomer to the porous membrane thereby to obtain a porous membrane having surface properties corresponding to that of the polymerized monomer. Unfortunately, the modified membranes so-produced exhibit properties which are undesirable for use with certain materials. Thus, the membrane so-produced oftentimes is colored, that is, a nonwhite color, and gives off colored, extractable compositions during use. Furthermore, the membranes have a tendency to adsorb proteins from solution and therefore are unacceptable in some applications such as in clinical diagnostic assays and immunofluorescent procedures. U.S. Pat. 4,618,533 discloses a process for modifying the surface characteristics of a porous membrane without plugging the pores of the membrane. There is no disclosure of modifying the bulk properties of the substrate membrane.
Accordingly, it would be highly desirable, for example, to provide a composite membrane having both desirable bulk physical strength and chemical resistance while having desired surface properties different from the bulk properties. Furthermore, it would be desirable to provide a membrane which is not absorptive to light and is microscopically transparent by virtue of surface modification thereof, which is characterized by very low levels of extractables and which exhibits very low adsorptivity for proteins. In addition, it would be desirable to provide a means for rendering a normally translucent or opaque membrane transparent without interfering with cell growth and viability.